Monday, April 12, 2010

Rare Earth, by Peter Ward and Donald Brownlee

Rare Earth: Why Complex Life Is Uncommon in the UniverseRare Earth is one of those books that just takes time to work your way through. The authors have managed to tone down the technical terms so that most reasonably literate people can still enjoy it and understand the concepts.
After the Copernican Revolution, most astronomers discounted the notion that Earth was all that exceptional within our universe. Sagan and others in the twentieth century postulated that there are many planets orbiting many solar systems which can support life, and that we should expect a high number of them to contain intelligent life. Ward and Brownlee began to look at some of the facts, and set forth the Rare Earth Theory, which says that the number of habitable planets is probably much smaller than previously thought.
They believe that life, in its more primitive forms, may indeed be widespread throughout the solar system, galaxy and universe, but that the conditions conducive to the evolution of metazoans, or complex life, are quite rare. One of the forms of primitive life that I'd heard mentioned before but not known much about are the extremophiles, microorganisms which live in places like the volcanic steam vents on the ocean floor, and others which can live within the rocks of the Earth, itself; they've been discovered down to depths of 3.5 km. Exremophiles adapted to different environments could live on other planets within our solar system, or anywhere else, for that matter.
As far as more complex life is concerned, it doesn't survive well at temperature extremes, or in places where liquid water and nutrients are not available. Planets must orbit within something called the Habitable Zone, which, for a sun like ours, is between .95 and 1.15 AUs. Our sun is gradually getting brighter, and eventually the Earth will grow too warm to support life, but we'll probaby be long gone by then, so not to worry.
I'd heard in my science classes that the Earth once had a tropical environment, where the dinosaurs thrived, but I hadn't previously been aware that there have been at least two times when there was global glaciation, which would have killed off many life forms. They call these time periods Snowball Earth.
In animal taxonomy, there is a division called phylum, representing "body form". According to the fossil record, virtually all of the phyla which exist today originated no later than the end of the Cambrian period. No new phyla have appeared in the fossil record since that time, and the number has actually declined from approximately 100 to the 40 recognized today. The sudden appearance of all of these phyla is called the Cambrian Explosion, and its cause is a great mystery to evolutionary biologists.
In addition the the mass extinctions caused by Snowball Earth events, the planet has also been struck by large comets or asteroids more than once. It is one of these events that is thought to have caused the dinosaurs to pass from the scene at the end of the Mezozoic era.
There's a lot of time spent in the book discussing plate tectonics, and their role in regulating climate. There seems to be a feedback loop involved in the level of atmospheric CO2, one of the greenhouse gases (actually water vapor is another, and it's about ten thousand times more plentiful, interestingly). "The most important element in reducing atmospheric carbon dioxide is the weathering of minerals known as silicates, such as feldspare and mica". As a planet warms, the rate of weathering increases, removing carbon dioxide from the atmosphere, which cools the planet. As the planet cools, the amount of CO2 available for weathering decreases, slowing the process, and the CO2 levels rise again, causing warming to occur. The Earth's temperature oscillates between warmer and cooler as a result.  Plate tectonics takes the weathering products from the sea floor through subduction, and these compounds are heated underground, releasing CO2 into the atmosphere again through volcanic activity.
The Moon and Jupiter also play roles in making Earth habitable, evidently. The size and orbit of the Moon keep Earth's axial tilt steady, so that our overall climate is more moderate than it would be if we had either more tilt or more wobbling. Jupiter, a huge gas giant, is believed in the past to have capture many comets and asteroids that would have otherwise made there way into the inner solar system, so that Earth has not been impacted more frequently by them, so we've had very few mass extinctions compared to what planets without the shield of a gas giant would experience.
This is an extremely interesting book about our planet and its place in the Universe. I really can't do it justice in a short review such as this.

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